The present invention relates to a communications system having repeater stations and more particularly, to a system for testing or evaluating such repeaters.
A type of communications system which uses repeater stations spaced at nominally 6,000 foot distances is termed a T1 pulse code modulated (PCM) carrier system and is used in the telephone industry. It is well known that on two pairs of wires such system provides 24 channels of data transmission. When a cable type transmission system is used, the conductor pairs employ repeaters to regenerate pulse code modulated signals. The repeaters are normally unattended necessitating remote testing means for routine maintenance.
Various methods have been devised for remote evaluation of repeater performance. For example, special code generators are used and test signals are transmitted over a repeater line. Such test signals contain audio frequencies and frequency selective filters located at each repeater station permit a particular repeater to be evaluated by proper choice of a test frequency. Such audio frequencies or test frequencies as will be described in detail below are generated by trio pulse groups of alternating polarity.
Advances in technology of cables has permitted increased distances over which repeaters can operate. While the distance itself between repeaters is not materially changed, it is the greater number of tandem repeaters which has been permitted to be employed which extends the effective length of the transmission system.
The test signals of the prior art, however, are not compatible with the increased quantities of repeaters through which the signal must be passed. The prior art signals tend to deteriorate when passing through each repeater. Consequently, the repeaters at the greatest distance cannot be, or are only marginally tested.
Furthermore, a typical prior art system of fault detection did not permit testing from one end of the transmission system only. Generally, while the prior art repeaters are two way transmission devices they have a combined output to a single frequency selective filter which is used for test purposes. Thus, test signals in most cases are permitted in only one direction at a time to avoid the ambiguity of two simultaneous outputs. Some modifications have been proposed. Repeaters which are now in service cannot practically be modified.
Another recent development which a test system thus far described must contend with are violation monitor and removal circuits (VMR) which are being added to PCM transmission systems to isolate errors to a single span, thus making the spans independent of each other. Such devices insure a perfectly sequential bipolar PCM signal. However, the VMR devices will also inherently modify existing test signals to make them useless for test purposes thereafter.